1. Field of The Invention
This invention generally relates to electrically conductive polymer compositions and is particularly concerned with the use of such compositions in the manufacture of electronic devices.
2. Description of The Prior Art
Certain polymers, particularly polyaniline have attracted intense interest in their electrical properties. While these properties have great potential for adapting polyaniline to uses in molecular electronic devices, batteries, chemically modified electrodes and sensors, their insolubility, poor physical strength, and their chemical reactivity gives rise to such poor physical film characteristics that the use of these materials, particularly polyaniline, for manufacturing electronic devices has been severely inhibited.
Polyaniline is usually obtained by electrochemical polymerization. In fact, Wrighton, et. al. (Journal of the American Chemical Society, Vol. 106 pages 7389-7396, 1984) disclosed electrochemically polymerizing several different types of conducting polymers including polyaniline onto substrates. Wrighton furthermore demonstrated transistor action in the polymers. However, electrochemical polymerization onto substrates is limited because it requires a conductive substrate, and the morphology of the films are inconsistent.
One alternative, chemical polymerization onto substrates, has proven difficult to control and disadvantageously exposes the substrate to unwanted contaminants.
A. G. MacDiarmid, et. al. (Molecular Crystals and Liquid Crystals (1985) Volume 125, pages 309-318) disclosed that when aniline is polymerized using hydrochloric acid solution as the oxidants, a conducting polymer is obtained. The resulting polymer has the following formula: ##STR1## This conducting polymer is green in color. Although polyaniline, made conductive with hydrochloric acid, has been employed in batteries (French Patent #1519729), it is too insoluble in organic solvents to be applied to substrates in any other fashion than electrochemical deposition, which of course, again, requires a conductive substrate. However, A. G. MacDiarmid, et. al. (Synthetic Metals, Volume 21 (1987) pages 21-30) did disclose casting films made from the insulating form of polyaniline dissolved in acetic acid. The insulating form of polyaniline is produced by treating the polyaniline with base to provide the following formula: ##STR2## This form of polymer takes on a blue color. U.S. Pat. No. 4,822,638 by Yaniger describes fabricating electronic devices from such insulating forms of polyaniline after doping the polymer with hydrochloric acid in order to chemically convert it to its conducting form after the insulating form has been coated onto the substrate.
In addition to the difficulty of controlling the polymerization and in addition to the potential exposure to unwanted contaminants, the insulating form is not substantially more soluble than the conductive form. Therefore, it was necessary to evaporate off large volumes of solvent from a very dilute solution. The remaining film is quite fragile and brittle, easily crumbling into a powder.
Accordingly, attempts have been made to produce the conducting form of polyaniline which could be applied to nonconductive substrates directly from solution.
Recently, Heeger (Synthetic Metals, Volume 32 (1989), page 263) disclosed a method for spin-coating polyaniline from sulfuric acid solution to form a conducting film. However, the method has a commercial drawback, in that, films spun from such a solvent will corrode metal substrates.
U.S. Pat. No. 4,798,685 by Yaniger disclosed polyanilines doped with electron acceptors such as alkyl halides to produce conductive polymers which were more soluble, more processable and which did not diffuse over time as compared to the hydrochloric acid polyaniline conducting polymers. Also U.S. Pat. No. 4,851,487 by Yaniger and Cameron disclosed anhydride-doped polyanilines. These recent polymers were also more soluble and thus more processable but, like Yaniger's earlier polymers, still fail to provide the film integrity needed to produce commercially acceptable electronic devices.
A polyaniline type conducting polymer, sufficiently soluble in organic solutions to be spun or cast onto either metal or polymer substrates as thick and commercially acceptable films, from which resistors and transistors or other electronic devices can be manufactured without loss of conductivity would represent a substantial advancement in the art and would satisfy a longfelt need.